1. Field of the Invention
This invention relates to the molding of an organic material and, in particular, to the molding of thermosetting polymers.
2. Art Background
Compositions denominated thermosetting resins are employed in a wide variety of products. For example, to provide protection thermosetting resin compositions (e.g., a novolac epoxy polymer or a cresol-novolac epoxy polymer either one combined with a phenol novolac or cresol-novolac hardener) are molded to encapsulate integrated circuit chips and their associated electrical leads. Generally to accomplish this encapsulation, a multi-cavity mold is employed to simultaneously process a large number, e.g., seventy or more, integrated circuits. These molds separate into halves and generally include (1) a common area for introducing the thermosetting resin, (2) channels, i.e., runners, which each run from the common area and which each provide access to one or more, (3) gates that open into (4) mold cavities. In the example of the simultaneous encapsulation of a plurality of chips, each chip and its associated electrical leads and connections are placed in a separate cavity, the mold is heated, thermosetting resin preheated to a temperature below the mold temperature is introduced into a reservoir, and pressure is applied to force the material (1) from the reservoir to the mold common area and (2) through the runners to the gates. This pressure is maintained until the cavities are filled and the resin has cured. To achieve an economical production rate, the entire mold-filling sequence is generally performed in from 10 to 30 seconds. After a suitable residence time to complete the curing reaction, e.g., 2 to 4 minutes, the mold is split and the molded objects, e.g., the encapsulated chips, are removed.
The viscosity of the resin strongly influences the molding procedure. If the thermosetting resin is too viscous at molding temperatures, it cannot be forced through the runners and gates without the use of a relatively high pressure. However, the use of relatively high pressures often has significant adverse effects. For example, in the case of electronic components, if a relatively high pressure is employed, the force of the polymer exerted on the contents of the cavity is often sufficient, for example, to damage the electrical connections to the integrated circuit.
It is generally believed that during nominal molding filling times resin viscosity does not substantially vary and remains at a level where a nominal pressures at the gate are produced. To determine a suitable molding pressure for a particular thermosetting resin and its associated viscosity, a spiral flow test is generally utilized. (See, for example, American Society for Testing Materials, Test D3123 described in ASTM Standards, Part 35.) In such a test, a mold having a spiral runner pattern but no gates or cavities is employed. The mold is heated to a typical molding temperature, e.g., to a temperature in the range 150 to 180 degrees C., and the resin is forced at a predetermined pressure through the spiral runner until, due to curing, it solidifies. The distance along the spiral that is traversed is considered to be a measure of the viscosity of the resin, and through empirical results, is related to a suitable molding pressure. The flow time in the spiral is similarly related to the time for gelation. Although molding procedures determined through the spiral flow test lead to acceptable yields for molded products, such as molded electronic components, improved yield is certainly desirable.